1. Field of the Invention
This invention relates to a powdered facing agent for use in forming a shell mold, and more particularly to a powdered facing agent composed of a powdered refractory, such as silica, alumina, zircon, silex, magnesia, etc., a novolak type phenol resin, a resol type phenol resin and/or vinyl acetate resin.
2. Description of the Prior Art
In forming a shell mold, it is a usual practice to coat a facing agent on the surface of a metal die for the purpose of obtaining a fine casting surface. Therefore, this facing agent is required to have properties that it is softened by heating, that it well adheres to a metal die when it is coated thereon, and that it loses its adhering force to the metal die when resin coated sand is brought to back up, while it well adheres to a shell mold made of the resin coated sand backed up, particularly in case that it is parted away from the metal die.
The facing agent widely used at present is of the type which is composed of a powdered silica mixed with, for instance, a novolak type phenol resin used as a thermosetting agent and hexamethylenetetramine used as a hardening agent. The facing agent of this type has a good adherence property to the metal die. However, it has disadvantages that the adherence thereof to the shell mold is poor when it is parted away from the metal die, and accordingly a coated film formed by coating the facing agent onto the metal die tends to remain on the metal die. Additionally, even when it has adhered to the shell mold, the coated film tends to peel off from the shell mold during pouring.
In order to eliminate the above-described disadvantages of the conventional facing agent, we heretofore proposed a novel facing agent composed of a powdered refractory, such as silica, alumina, zircon, silex, magnesia, etc., a novolak type phenol resin, and a small amount of vinyl acetate resin, which has a good adherence property to resin coated sand as a back-up material and has a good parting property from the metal die, as disclosed in U.S. Patent Application Ser. No. 362,240 filed May 21, 1973 (now abandoned). However, through our further careful study, we found that the facing agent of this type also has the following disadvantages:
At the portions where the coated film is too thick, unhardened portions tends to occur, thereby weakening the adhering force of the coated film to the backed up resin coated sand, with a result that, the coated film is partially left on the metal die. On the other hand, at the portions where the temperature of the metal die is too low, unhardened coated film also tends to occur, with a result that, the coated film tends to be left on the metal die. Accordingly, a temperature control must be carried out so that the difference of the temperature of the metal die may be minimized.
In the case of the facing agent containing a novolak type phenol resin, a hardening rate control for the coated film is difficult even if vinyl acetate resin is admixed therewith. When hexamine is admixed with the facing agent in order to control the hardening rate of the coated film, its reaction becomes too rapid. As a result, veining tends to occur and the adhesion of the coated film to the backed up resin coated sand tends to be deteriorated.